It hasn't been officially updated or supported for almost 20 years although there have been some independent software teams that have tried to keep it viable since then... (at least until 2022/3). So I doubt too many people use it especially considering how unsafe it might be to run any software that isn't continually updated for online safety issues.

https://en.wikipedia.org/wiki/Eudora...lient)#History

What did you want to know/ask about it?